Immersion lithography is known. The immersion medium can be matched to an optical element, which is the last one in the direction of the beam path through the immersion lithography objective, that is to say in the event of deformations or movements of this last optical element the immersion medium follows the same facing surface of the optical element. Since the refractive index of the immersion medium and of the last optical element are mostly similar, the errors of the last optical element are to be interpreted as polishing errors of the matchings in the event of deformations, as wedginess errors of a lens in the event of instances of tilting, and as central thickness errors of a lens in the event of a displacement along the optical axis.
Such deformations or displacements of the last optical element along the optical axis, as well as instances of tilting of the element can be caused, in particular, by dynamic changes in the ambient conditions. Thus, for example, pressure changes in the surroundings relative to the gas pressure in the interior of the objective can effect a movement of the last optical element. Such a pressure change can even be caused by opening a door in the room in which the lithography objective is standing. Above all, when relatively quick or short term changes are involved, a control system present at the objective may be incapable of undertaking regulation designed to enable the last optical element otherwise to retain its exact position.
Furthermore, even slight movements in position of the wafer, which is arranged opposite the last optical element, or the high speeds of the element as it is being adjusted can lead to dynamic pressure variations within the immersion medium that influence the position of the last optical element in turn.
This can be a particular problem when such changes in position of the last optical element occur during the exposure, since this can give rise to imaging errors that are mostly not tolerable. For this reason, positional errors of the last optical element should be avoided or greatly limited, as far as possible.